A bio-economic model for regional-scale weed management: Nassella neesiana (Chilean needle grass) in New Zealand as a case study

We describe a bio-economic model for Nassella neesiana (Chilean needle grass) that estimates the net benefit of a containment programme for the weed in Canterbury as the difference between the cost of containment and the costs incurred over time should the weed spread. Logistic spread is assumed with the maximum area that could be invaded (772,080 ha) determined by constraining a climate niche model for the weed to susceptible farm-system types within productive land-use-capability classes. The current size of the invaded area (80 ha) was determined by field observations in Canterbury in 2008, and the spread rate (201 years to 90% saturation) was derived from observations in the adjacent Marlborough region. With these assumptions and costs discounted at 8% per year over 100 years, the net benefit is negative $173,178 ($83,900 - $257,078) and containment would not be economically worthwhile. The opposite conclusion is reached for a range of lower discount rates and higher spread rates. The modelling methods presented here may be considered as a generic blueprint for evaluating proposed regional-scale weed management programmes.